Pressed down specific keyboard key.
string|int $char could be either char ('b') or char-code (98):
KeyModifier::*|null $modifier keyboard modifier (could be 'ctrl', 'alt', 'shift' or 'meta'):
void
public function keyDown($char, ?string $modifier = null) { $this->getDriver() ->keyDown($this->getXpath(), $char, $modifier); }